[libpng15] Added set(CMAKE_CONFIGURATION_TYPES ...) to CMakeLists.txt

(Andrew Hundt)
[libpng15] Deleted set(CMAKE_BUILD_TYPE) block from CMakeLists.txt
[libpng15] Updated README (removed CompuServe, added simplesystems.org)
[libpng15] Make ARM NEON support work at compile time (not just configure time).

This moves the test on __ARM_NEON__ into pngconf.h to avoid issues when
using a compiler that compiles for multiple architectures at one time.
[libpng15] Port ARM support changes from libpng-1.6.3beta07.